In the lost wax casting process a wax pattern of a component is produced. The wax pattern is a replica of the component to the produced. Usually a number of wax patterns are assembled together on a wax gating tree to form a cluster or wax mould assembly. The wax mould assembly is immersed in a liquid ceramic slurry which quickly gels after draining, strengthening refractory granules are sprinkled over the ceramic slurry covered wax mould assembly and the refractory granules bond to the slurry coating to produce a ceramic layer on the wax mould assembly. This process is repeated several times to produce many ceramic layers which have a total thickness of about 1/4 inch (6 mm) to 1/2 inch (12 mm) on the wax mould assembly. The wax is then melted out leaving a ceramic shell mould having an internal cavity identical in shape to that of the original wax cluster. This ceramic shell mould is called an investment casting mould. The mould is fired at a high temperature between 950.degree. C. and 1100.degree. C. to purify it by removing all traces of residual wax, while at the same time curing the ceramic shell mould. The ceramic shell mould is then transferred to a casting furnace, which may be operated at either vacuum conditions or at atmospheric conditions. A charge of molten metal is then poured into the ceramic shell mould and the mould is allowed to cool to room temperature, after which the ceramic shell mould is removed leaving the cast component or components.
In the lost wax casting of hollow turbine blades or turbine vanes, the wax patterns of the hollow turbine blades or turbine vanes are produced by injecting wax into a pattern die which has one or more preformed ceramic cores located therein. The pattern die has convex and concave aerofoil shaped surfaces and the ceramic core is spaced from the convex and concave shaped surfaces of the pattern die by chaplets to ensure the correct thickness gap exists between the surfaces of the die and the ceramic core surfaces. The ceramic core has shaped projections which locate in correspondingly shaped apertures in the pattern die. The ceramic core is prevented from moving longitudinally in the pattern die by a precisely positioned pin and slot arrangement.
It is difficult to optimise the position of the ceramic core relative to the pattern die surfaces due to the manufacturing tolerances of size and shape of the ceramic core and also because of distortions within the ceramic core making process. It is particularly difficult to optimise the trailing edge position of the ceramic core relative to the pattern die surfaces due to the distortions of the ceramic core, because the relationship between the trailing edge and the shaped projections of the ceramic core suffers the greatest dimensional variations. The larger the ceramic core the more pronounced is the distortion.
The chaplets fitted to the core are positioned to ensure that the correct thickness of wax is achieved, but where distortion is excessive the point load exerted onto the ceramic core by the chaplet strains the ceramic core, because they are trying to correct the distorted shape against the restraint imposed by the shaped projections of the ceramic core locating in the corresponding shaped apertures in the die. In the extreme case the strain is enough to fracture the brittle ceramic core thus scrapping the wax pattern. If the strain is insufficient to break the ceramic core, there is a residual strain imposed in the ceramic core which, when the wax is removed from the ceramic shell mould, causes the ceramic core to spring back to its free state and subsequently produces a cast turbine blade or turbine vane which has a thin wall section.